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Abstract:

Methods, implantable medical devices and systems configured to perform
analysis of captured signals from implanted electrodes to identify
cardiac arrhythmias. In an illustrative embodiment, signals captured from
two or more sensing vectors are analyzed, where the signals are captured
with a patient in at least first and second body positions. Analysis is
performed to identify primary or default sensing vectors and/or templates
for event detection.

Claims:

1-9. (canceled)

10. An apparatus for use with an implantable medical device having
sensing circuitry, the apparatus comprising a user interface for
presenting instructions to a user and receiving inputs and communications
circuitry for communicating with the implantable medical device, the user
interface and communications circuitry configured for optimization of the
implantable medical device's sensing circuitry by performing the
following: the user interface presenting a screen instructing a recipient
of the implantable medical device to adopt a first posture and indicating
that it is awaiting a first input indicating that the recipient has
adopted the first posture; upon receipt, with the user interface, of the
first input, the communications circuitry instructing the implantable
medical device to perform a first data capture sequence and awaiting an
indication from the implantable medical device that the first data
capture sequence is complete; following receipt of an indication from the
implantable medical device that the first data capture sequence is
complete, the user interface presenting a screen instructing the
recipient of the implantable medical device to adopt a second posture and
indicating that it is awaiting a second input indicating that the
recipient has adopted the second posture; upon receipt, with the user
interface, of the second input, the communications circuitry instructing
the implantable medical device to perform a second data capture sequence
and awaiting an indication from the implantable medical device that the
second data capture sequence is complete; and following receipt of an
indication from the implantable medical device that the second data
capture sequence is complete, the user interface presenting a screen
indicating that optimization is complete.

11. The apparatus of claim 10 wherein the apparatus is configured as a
programmer for clinical programming, and the implantable medical device
is an implantable cardiac stimulus device.

12. The apparatus of claim 10 wherein the apparatus is configured as a
home monitoring device for use by a patient in his or her home.

13. The apparatus of claim 10 wherein the communications circuitry is
further configured to direct selection of a cardiac sensing vector for
the implantable medical device in response to data captured while the
recipient is holding the first and second postures.

14. An implantable cardiac stimulus device configured for use with an
implantable lead having a plurality of electrodes, the implantable
cardiac stimulus device including operational circuitry having sensing
inputs for coupling to at least three electrodes such that at least first
and second sensing vectors are defined, the implantable cardiac stimulus
device being configured to perform a sensing vector configuration
sequence in which first and second sensing vectors selected as primary
and secondary sensing vectors, respectively, and the operational
circuitry is configured to perform the following while sensing cardiac
signals for the purpose of identifying an arrhythmia requiring therapy:
maintain an X/Y counter in which X represents a quantity of recently
detected cardiac cycles in which an arrhythmic condition is likely and Y
represents a total number of recently detected cardiac cycles, while
using the primary sensing vector to identify and analyze cardiac cycles
for purposes of maintaining the X/Y counter; and upon reaching a first
threshold for X, and before reaching a second threshold for X, using the
secondary sensing vector to identify and analyze cardiac cycles to
confirm whether an arrhythmic condition is likely.

15. The implantable cardiac stimulus device of claim 14 wherein the
operational circuitry is further configured to continue maintaining the
X/Y counter until reaching the second threshold for X and, if the cardiac
cycles detected in the secondary sensing vector confirm that an
arrhythmic condition is likely, determining that therapy is warranted.

16. The implantable cardiac stimulus device of claim 14 wherein the
operational circuitry is configured such that: the sensing vector
configuration sequence comprises establishing a first template for use in
analyzing detected cardiac cycles in the primary sensing vector; the
sensing vector configuration sequence comprises establishing a second
template for use in analyzing detected cardiac cycles in the secondary
sensing vector; the step of using the primary sensing vector to identify
and analyze cardiac cycles for purposes of maintaining the X/Y counter
includes using the first template to distinguish normal cardiac cycles
from arrhythmic cardiac cycles; and the step of using the secondary
sensing vector to identify and analyze cardiac cycles for purposes of
maintaining the X/Y counter includes using the second template to
distinguish normal cardiac cycles from arrhythmic cardiac cycles; wherein
the operational circuitry is configured to determine that therapy in
response to an arrhythmia is warranted if neither the primary sensing
vector and first template, nor the secondary sensing vector and second
template template indicate normal cardiac activity.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. patent application Ser.
No. 13/491,529, filed Jun. 7, 2012, which is a divisional of U.S. patent
application Ser. No. 11/672,353, filed Feb. 7, 2007, now U.S. Pat. No.
8,200,341 and titled SENSING VECTOR SELECTION IN A CARDIAC STIMULUS
DEVICE WITH POSTURAL ASSESSMENT, the disclosure of which is incorporated
herein by reference.

[0002] This application is related to U.S. patent application Ser. No.
11/623,472, filed Jan. 16, 2007 and titled SYSTEMS AND METHODS FOR
SENSING VECTOR SELECTION IN AN IMPLANTABLE MEDICAL DEVICE USING A
POLYNOMIAL APPROACH, now U.S. Pat. No. 7,783,340, the disclosure of which
is incorporated herein by reference. This application is also related to
U.S. patent application Ser. No. 11/441,522, filed May 26, 2006,
published as US Patent Application Publication Number 2007-0276445 and
titled SYSTEMS AND METHODS FOR SENSING VECTOR SELECTION IN AN IMPLANTABLE
MEDICAL DEVICE, the disclosure of which is incorporated herein by
reference.

FIELD

[0003] The present invention relates to the field of implantable medical
devices. More particularly, the present invention relates to implantable
devices that monitor and/or stimulate the heart.

BACKGROUND

[0004] Implantable cardiac monitoring and/or stimulus devices can provide
various benefits to patients who receive them. Such devices are adapted
to monitor cardiac activity of a patient while implanted and, if so
equipped, to provide stimulus when necessary to assure adequate cardiac
function. New and different methods are desired for configuring and
performing cardiac signal assessment in such devices.

SUMMARY

[0005] The present invention, in an illustrative embodiment, includes an
implantable medical device that includes sensing electrodes and circuitry
that allow the device, when implanted in a patient, to sense electrical
activity emanating from the patient's heart along a plurality of sensing
vectors. In the illustrative embodiment, the implantable medical device
is configured to select a primary or default sensing vector by observing
cardiac signal characteristics along one or more of the plurality of
sensing vectors. In an illustrative embodiment, observation of the
cardiac signal characteristics includes initialization in terms of the
body position or posture of the patient.

[0006] In another illustrative embodiment, a device as described above is
included as a part of a system including an external programmer, wherein
the programmer and implanted device are configured to communicate with
one another. In this embodiment, the system is configured such that the
patient may be directed to perform certain acts and/or assume selected
postures/poses/body positions via the programmer, allowing the implanted
device to observe the effects of changes of posture by the patient on
captured cardiac signal. The implanted device (or the programmer,
depending upon the particular configuration) may then select a primary or
default sensing vector.

[0007] Another illustrative embodiment includes a method of selecting a
vector for use in sensing cardiac events. In the illustrative method,
sensing characteristics along several vectors may be considered with the
patient in various body positions (for example, standing, sitting, and/or
lying down). Using the captured sensing characteristics, a default or
primary sensing vector may be selected. In an illustrative example, the
method includes directing the patient to assume a set of
postures/poses/body positions.

[0008] In addition to selecting a primary or default sensing vector, in
some embodiments, a secondary vector is selected for various uses.

[0014] FIGS. 8A-8E are graphical representations of display outputs of a
programmer during an illustrative method of postural assessment.

DETAILED DESCRIPTION

[0015] The following detailed description should be read with reference to
the drawings. The drawings, which are not necessarily to scale, depict
illustrative embodiments and are not intended to limit the scope of the
invention.

[0016] The present invention is related to U.S. patent application Ser.
No. 11/441,522, filed May 26, 2006 and entitled SYSTEMS AND METHODS FOR
SENSING VECTOR SELECTION IN AN IMPLANTABLE MEDICAL DEVICE, published as
US Patent Application Publication Number 2007-0276445, the disclosure of
which is incorporated herein by reference. In particular, the '522
Application shows illustrative methods of analyzing cardiac signals
captured along a given sensing vector. The methods shown therein are
illustrative of analytical methods and "scoring" that may also be
performed in association with the present invention.

[0017] FIGS. 1A-1B, respectively, show subcutaneous and transvenous
implanted cardiac stimulus systems relative to the heart. Referring to
FIG. 1A, the patient's heart 10 is shown in relation to an implanted,
subcutaneous cardiac stimulus system including a canister 12. A lead 14
is secured to the canister 12 and includes sensing electrode A 16, coil
electrode 18, and sensing electrode B 20. A can electrode 22 is shown on
the canister 12. Several vectors for sensing are therefore available
including A-can, B-can, and A-B. It should be noted that the use of the
coil electrode 18 as a sensing electrode is also possible. Illustrative
subcutaneous systems are shown in U.S. Pat. Nos. 6,647,292 and 6,721,597,
and the disclosures of these patents are incorporated herein by
reference. Some embodiments include a unitary system having two or more
electrodes on a housing as set forth in the '292 patent, rather than that
which is shown in FIG. 1A. A unitary system including an additional lead
may also be used. It should be understood that for any vector discussed
herein, either of the two available polarities for each vector is
possible and may be analyzed and/or selected, if desired.

[0018] Referring now to FIG. 1B, a transvenous system is shown relative to
a patient's heart 30. The transvenous cardiac stimulus system includes a
canister 32 connected to a lead 34. The lead 34 enters the patient's
heart and includes electrodes A 36 and B 38. The illustrative example
also includes coil electrodes 42, shown both internal and external to the
heart 30. The coil electrodes 42 may be used for sensing or stimulus
delivery. In the illustrative example, electrode A 36 is located
generally in the patient's ventricle, and electrode B 38 is located
generally in the patient's atrium. The lead 34 may be anchored into the
patient's myocardium. Again, a can electrode 40 is shown on the canister
32. With the transvenous system, plural sensing vectors may be defined as
well.

[0019] In both FIGS. 1A and 1B, one or more sensing electrodes may also be
used for stimulus delivery. Some embodiments of the present invention may
be used in combination systems that may include sensing vectors defined
between two subcutaneous electrodes, a subcutaneous electrode and a
transvenous electrode, or two transvenous electrodes. For example, the
present invention may be embodied in a hybrid system having electrodes
for each of several transvenous, epicardial, and/or subcutaneous
locations.

[0020] In the configurations of FIGS. 1A and 1B, there are multiple
sensing vectors available. Detection of cardiac function along one of
these sensing vectors allows the implanted cardiac stimulus system to
determine whether treatment is indicated due to the detection and
identification of a malignant condition such as, for example, a
ventricular tachycardia. An implanting physician may perform vector
selection by determining which of the captured vectors is best, for
example by visual inspection of a graphical representation of captured
signals. However, this requires an assessment of cardiac function along
several vectors and may increase the time needed to perform implantation,
and also increases the risk of human error. Further, the selection of a
vector may require advanced or specialized training, as selection of a
suitable vector among those available is not necessarily intuitive.

[0021] Robust sensing vector selection methods are desirable, as well as
devices adapted to perform such methods. The present invention, in
illustrative embodiments, provides such methods and uses various criteria
for doing so. Some embodiments include implantable devices and
programmers for implantable devices that are adapted to perform such
methods.

[0022] The systems shown in FIGS. 1A-1B may include operational circuitry
and power sources housed within the respective canisters. The power
sources may be, for example, batteries or banks of batteries. The
operational circuitry may be configured to include such controllers,
microcontrollers, logic devices, memory, and the like, as selected,
needed, or desired for performing the illustrative methods set forth
herein. The operational circuitry may (although not necessarily) further
include a charging sub-circuit and a power storage sub-circuit (for
example, a bank of capacitors) for building up a stored voltage for
cardiac stimulus taking the form of cardioversion and/or defibrillation.
The operational circuitry may also be adapted to provide a pacing output.
Each of cardioversion/defibrillation and pacing sub-circuitry and
capacities may be incorporated into a single device. The methods
discussed below may be embodied in hardware within the operational
circuitry and/or as instruction sets for operating the operational
circuitry and/or in the form of machine-readable media (optical,
electrical, magnetic, etc.) embodying such instructions and instruction
sets.

[0023] Each of the devices 12, 32 may further include such components as
would be appropriate for communication (such as RF communication or
inductive telemetry) with an external device such as a programmer. To
this end, programmers 24 (FIG. 1A) and 42 (FIG. 1B) are also shown. For
example, during an implantation procedure, once the implantable device
12, 32 and leads (if included) are placed, the programmer 24, 42 may be
used to activate and/or direct and/or observe diagnostic or operational
tests. After implantation, the programmer 24, 42 may be used to
non-invasively determine the status and history of the implanted device.
The programmer 24, 42 and the implanted device 12, 32 may be adapted for
wireless communication allowing interrogation of the implanted device in
any suitable manner for an implanted device system. The programmers 24,
42 in combination with the implanted devices 12, 32 may also allow
annunciation of statistics, errors, history and potential problem(s) to
the user or physician.

[0024] FIG. 2 is a block diagram illustrating steps in an illustrative
implant procedure. From a start block 100, the first step is the physical
implantation itself 102, which may include various surgical preparations
as are known in the art, incision of the patient, and emplacement of a
system, for example, a transvenous or subcutaneous system as shown above
in FIGS. 1A-1B. Methods for physically implanting a transvenous device
are well known. A subcutaneous device may be implanted, for example, as
set forth in copending U.S. patent application Ser. No. 11/006,291, filed
Dec. 6, 2004 and titled APPARATUS AND METHOD FOR SUBCUTANEOUS ELECTRODE
INSERTION, now U.S. Pat. No. 7,655,014; and/or copending U.S. patent
application Ser. No. 11/497,203, filed Aug. 1, 2006, published as US
Patent Application Publication Number 2008-0046056 and titled ELECTRODE
INSERTION TOOLS, LEAD ASSEMBLIES, KITS AND METHODS FOR PLACEMENT OF
CARDIAC DEVICE ELECTRODES, the disclosures of which are incorporated
herein by reference.

[0025] With the system in place in the patient, the device is initialized,
as indicated at 104. This may include various functions as indicated at
106, such as power-up of the device, system check, lead connection,
detection and impedance checks. Initialization 104 may also include
vector selection steps and template formation steps, allowing an initial
set-up of the device while the patient is in the operating room.
Illustrative methods of template formation are discussed in copending
U.S. patent application Ser. No. 10/999,853, filed Nov. 29, 2004 and
entitled METHOD FOR DEFINING SIGNAL TEMPLATES IN IMPLANTABLE CARDIAC
DEVICES, now U.S. Pat. No. 7,376,458, the disclosure of which is
incorporated herein by reference. Illustrative methods of vector
selection are discussed in copending U.S. patent application Ser. No.
11/441,522, filed May 26, 2006, published as US Patent Application
Publication Number 2007-0276445 and titled SYSTEMS AND METHODS FOR
SENSING VECTOR SELECTION IN AN IMPLANTABLE MEDICAL DEVICE; copending U.S.
patent application Ser. No. 10/901,258, filed Jul. 27, 2004 and entitled
MULTIPLE ELECTRODE VECTORS FOR IMPLANTABLE CARDIAC TREATMENT DEVICES, now
U.S. Pat. No. 7,392,085; and U.S. Pat. No. 6,988,003 entitled OPTIONAL
USE OF A LEAD FOR A UNITARY SUBCUTANEOUS IMPLANTABLE
CARDIOVERTER-DEFIBRILLATOR, the disclosures of which are incorporated
herein by reference. The inclusion of each of functions 106 may vary
depending upon the particular device.

[0026] After device initialization at 104, the device is tested as
indicated at 108. For an implantable cardioverter defibrillator (ICD),
for example, fibrillation may be induced in the patient in order to
determine whether the ICD accurately senses the fibrillation and
successfully delivers therapy that returns the patient to normal cardiac
rhythm. If testing is unsuccessful, or if the device does not initialize,
the system may be explanted. If, instead, the initialization at 104 is
completed successfully and the system passes device testing at 108, then
the surgical portion of the implantation is completed, as shown at 110,
with appropriate methods including closing incisions made during
implantation, etc.

[0027] In the illustrative method, postural analysis 112 follows
completion of the surgical portion of the implantation method. During
postural analysis 112, after the patient has had the device inserted and
activated, and, possibly, after the patient has had time to recuperate
somewhat, the operation of the implanted device is observed and may be
modified. In particular, the patient may be asked to assume a series of
positions, for example, sitting down and then standing up, while the
implanted device gathers data to determine which of its available sensing
vectors is best suited to permanent operation. In one illustrative
example, the patient is led through a series of body positions such that
the device may determine a single sensing vector for use as a primary or
default sensing vector all the time, such that changes in sensing
operation do not have to occur whenever the patient changes posture or
body position.

[0028] In another illustrative example, an optimal or best vector is
determined for each of the several body positions, and body position is
monitored during operation such that the implanted device may select the
optimal vector for a patient's current body position. Body position may
be monitored, for example, by the provision of physical sensors that
detect body position by reference to gravity, to movements, or the like.
Transthoracic impedance can be used to provide a measure of patient body
position, for example. An activity sensor may be used to infer whether
the patient is standing versus lying down. Alternatively, body position
may be monitored by observation of captured electric signals. For
example, during postural analysis 112, the system may be configured to
identify signal markers to differentiate cardiac signals captured while
the patient is in each of several body positions and, thereafter, to
identify the patient's body position by observation of the patient's
cardiac signals.

[0029] Once postural analysis 112 is complete, the patient may then be
discharged 114, as the implantation procedure and process is then
complete. Following discharge 114, the patient may be requested to return
for further diagnostics, for example, initialization may be updated (such
as functions 106) and the postural analysis 112 may be later repeated.
This may be done, for example, as the patient's physiology changes due to
reaction to the implantation itself, with changes in patient medication,
and/or as the patient ages.

[0030] FIGS. 3-5 show illustrative methods of postural assessment in an
implanted medical device. FIG. 3 illustrates a confirmation approach to
postural assessment, in which a vector is identified on the basis of its
characteristics while the patient is in a given body position, and the
identified vector is then checked to confirm that it is usable regardless
of the body position of the patient. The method begins at block 130, with
the patient in a given body position, in this instance, supine. In other
embodiments, the patient may begin in a different body position, such as
prone, reclined, standing, or in whatever position the patient prefers to
sleep in, for example.

[0031] With the patient in the given position, a vector is selected, as
shown at 132. A vector may be selected at 132, for example, on the basis
of the signal-to-noise ratio (SNR) of cardiac signals captured along that
vector. Other metrics for selecting a vector at 132 may include signal
amplitude, noise amplitude, etc. In an illustrative embodiment, a
combination of SNR and signal amplitude are taken into consideration. For
example, a formula using both SNR and amplitude may be used. Some
illustrative examples of such analysis are shown in copending U.S. patent
application Ser. No. 11/441,522, filed May 26, 2006, published as US
Patent Application Publication Number 2007-0276445 and titled SYSTEMS AND
METHODS FOR SENSING VECTOR SELECTION IN AN IMPLANTABLE MEDICAL DEVICE.

[0032] In another illustrative embodiment, a default vector is assumed,
rather than selected on the basis of any metric. For example, if a
particular system and implantation has a vector that works for most
patients, then that vector may be selected first to start analysis. In
another example, there may be a preferred vector, such as where there are
two sensing-only electrodes and one or more sensing/shocking electrodes.
Due to possible physical changes at the electrode/tissue interface for
the sensing/shocking electrodes when stimulus is delivered, the vector
between the two sensing-only electrodes may be "preferred". Thus, in
starting the method, the "preferred" vector may be selected at step 132.

[0033] With the vector selected at 132, a template for analysis of cardiac
signals may optionally be formed, as shown at 134. This optional step of
the method of FIG. 3 may include capturing a set of signals with the
selected vector, identifying likely cardiac events in the set of signals,
and forming windows of captured signals around fiducial points likely to
correspond to cardiac events.

[0034] An illustrative example of template formation may be as follows.
First, the signal captured along the selected vector may be compared to a
threshold and, when the threshold is exceeded, a cardiac event is assumed
to be likely. A peak value in a set of samples following the threshold
crossing (for example, the next 40 sequential samples, captured at 256
Hz) is identified as a fiducial point, and a window of samples around the
fiducial point is calculated. This initial capture is presumed to be a
cardiac event. Several additional "events" are subsequently captured in
association with later threshold crossings, with the fiducial point
and/or window being selected in a manner corresponding to that of the
original identified "event".

[0035] The captured events are then compared to one another, for example,
using correlation waveform analysis, or alternatively, another metric
such as QRS width, which may be identified as a duration in which the
signal remains above a minimum threshold. If the set of events is
sufficiently similar, then a template is formed by selecting a
representative event or by averaging a set of events. In some
embodiments, the template may be dynamically updated by averaging in
later-captured events. Additional examples of template formation may also
be found in copending U.S. patent application Ser. No. 10/999,853, filed
Nov. 29, 2004 and entitled METHOD FOR DEFINING SIGNAL TEMPLATES IN
IMPLANTABLE CARDIAC DEVICES, now U.S. Pat. No. 7,376,458. In some
embodiments, template formation may fail where similarity among sets of
captured events does not occur. For example, a time-out may be used, and,
if after a period of time (60-180 seconds, for example), a template
cannot be formed using similarity analysis, the vector under
consideration may be identified as poorly suited to detection.

[0036] Next, the patient is directed to change body positions, as
indicated at 136. One or more additional positions beyond the original
position may be used. For example, if the patient is supine, as indicated
at 130, the patient may be asked to move to a prone position, to sit up,
and/or to stand. The positions listed at 138 are merely illustrative, and
no set number of such positions is necessary. In an illustrative
embodiment, it is determined what positions the patient tends to rest in,
for example, reclined, and/or the patient's sleep position, such as on
the patient's side, in order that the posture assessment considers those
positions the patient is in the most. For each position the patient is
asked to assume, the implanted device analyzes the selected vector to
determine whether the selected vector will function adequately for
cardiac event detection and analysis. In an illustrative example, if the
selected vector fails to function adequately in any position, the method
skips any remaining body positions and advances to step 140. Otherwise,
the selected vector is analyzed at each of at least two positions.

[0037] In an illustrative example, the selected vector is analyzed in each
of the positions by considering one or more of SNR, signal amplitude,
and/or noise amplitude. For example, the SNR and signal amplitude may be
considered by the use of a formula, as further explained below.

[0038] The method then goes to step 140, where it is determined whether
the selected vector has been verified or confirmed as a "good" sensing
vector. If so, the method ends at 142. If the selected vector is not
verified or confirmed, the method returns to step 132 and selects a
different vector, with the patient being asked to again assume the first
body position upon return to step 132 and, thereafter, to move through
additional body positions as directed at step 136. In this method, a
vector is first selected and then tested as the patient changes body
positions.

[0039] FIG. 4 illustrates a deterministic approach, as contrasted to the
confirmation approach of FIG. 3. In FIG. 4, for example, the patient
assumes a given body position and holds that position until data is
gathered for each of the available vectors. The method of FIG. 4 begins
with start block 160. The patient is asked to select a body position, as
indicated at 162. Illustrative positions are listed at 164; in an
example, the patient is asked to lay supine and is later asked to sit
and/or stand. Each vector is then analyzed while the patient remains in
the selected body position, as shown at 166. After data is captured for
each vector during step 166, it is determined if all positions have been
tested, as indicated at 168. If not, the method returns to step 162,
where a different body position is selected and the patient is asked to
assume a different body position. Once each desired body position is
tested, the method exits the loop from 168, and a vector is selected as
indicated at 170. Once a vector is identified on the basis of the data
captured in the loop of 162-166-168, the method may perform template
formation as indicated at 172. In some embodiments, if template formation
172 fails, the method may return to step 170 and select a different
vector. The method then ends, as indicated at 174.

[0040] In an alternative embodiment, first and second vectors are
identified, with the second vector being a back-up vector for use either
because the first, primary or default vector becomes unavailable or,
alternatively, for use if the first vector provides ambiguous indications
of whether the patient is experiencing an arrhythmia.

[0041] In some embodiments, a single vector is identified for use while
the patient is in any body position. In other embodiments, several
templates may be formed for the patient. For example, analysis of data
captured while the patient is in a supine position may indicate a
different "best" vector than that captured while the patient is an
upright position. Two templates could then be formed, without regard for
whether the first and second templates are formed using the same vector,
where each template includes information for its sensing vector
configuration.

[0042] Illustratively, the first template could use sensing vector A-Can
(see FIG. 1A) and could be defined at the time of postural assessment
while the patient is supine, and the second template could use vector A-B
(see FIG. 1A) and could be defined at the time of postural assessment
while the patient is sitting upright. Then, during analysis, the primary
template at any given time could be whichever indicates ordinary cardiac
function. If neither template indicates ordinary cardiac function, it may
be presumed that an arrhythmia is occurring, and a stimulus may be
delivered. This may be performed as part of a Boolean approach to finding
arrhythmic activity:

[0043] IF Analysis(A) fails

[0044] AND IF Analysis(B) fails

[0045] THEN Event indicates Malignant Rhythm

This tiered analysis may prevent misdiagnosis of the cardiac rhythm due
to a change of the patient's body position.

[0046] In another illustrative example, the primary and secondary
templates may be "switched" one for the other in response to an output
from a body position sensor. For example, if the primary template is
associated with a first body position and the secondary template is
associated with a second body position, an output of a body position
sensor may be monitored, and the templates switched depending upon the
body position indicated by the body position sensor.

[0047] Additionally, switching between the templates may be achieved
without needing the use of a dedicated position sensor. In an
illustrative example, only the Analysis(A) is performed in a
morphology-based analysis system until a first threshold of abnormality
is met. For example, if an X out of Y counter is used, the Analysis(A)
may be used by itself until a first X out of Y threshold is met. In an
illustrative example, if 18/24 events is the threshold for a
determination that a malignant cardiac rhythm is occurring, then an 8/24
threshold may be used to activate Analysis(B). If 2-3 events occur before
Analysis(B) begins functioning, then a smooth transition may occur if the
patient has changed positions since, by the time the 18/24 counter fills
for Analysis(A), 8 or more events will have been detected using
Analysis(B). In an illustrative example, if 4/8 of the events using
Analysis(B) are found to meet template comparison parameters, the system
may switch to Analysis(B), using a different template and/or vector than
Analysis(A) for primary analysis. In some illustrative embodiments that
use both primary and secondary templates, stimulus delivery (or
preparation for stimulus delivery) may be delayed until it is determined
that the analysis with each template indicates a malignant cardiac
rhythm.

[0048] The method of FIG. 4 calls for more data to be gathered than the
method of FIG. 3, regardless of whether one vector is superior or not.
Therefore, this method may be slower in some instances than that of FIG.
3. However, with the method of FIG. 3, the patient may be asked to
perform and repeat several movements during postural assessment. If a
device is implanted in a patient who is relatively weak, for example, due
to advanced congestive heart failure, repeated movements may be
undesirable. Illustrative examples may be configured to perform either
method. In another illustrative example, the devices (the implanted
medical device and/or the programmer) in the system are equipped to
perform either method, with the programmer allowing a physician to select
one method or the other.

[0049] FIG. 5 illustrates a detailed method for an illustrative
embodiment. As indicated at 200, the method begins with the patient (PT)
supine. Step 200 may be a directive given from the programmer to a
physician to have the patient assume a supine position, or it may be
given directly to the patient from the programmer itself. With this
position verified (for example, the programmer may request an input
indicating that the patient is in the requested position), a first vector
is selected, as indicated at 202.

[0050] A VS SCORESUP is then calculated, as indicated at 204. The VS
SCORESUP may be a "score" calculated to indicate the quality of the
sensing vector. As discussed above, this may include consideration of
signal amplitude, SNR, etc. Calculation of a SCORE may make use of a
formula, a look-up table, or any suitable method for placing a metric on
the quality of a sensing vector. Illustrative methods are shown in
copending U.S. patent application Ser. No. 11/441,522, filed May 26,
2006, published as US Patent Application Publication Number 2007-0276445
and entitled SYSTEMS AND METHODS FOR SENSING VECTOR SELECTION IN AN
IMPLANTABLE MEDICAL DEVICE.

[0051] After the score is calculated at 204, it is determined whether all
vectors have been analyzed, as indicated at 206. If not, a different
vector is selected, as shown at 208, and the method calculates another VS
SCORESUP at 204. If all the vectors have been considered at step
206, the method continues to block 210.

[0052] At block 210, the programmer or the implantable device, depending
on the system configuration, determines whether additional patient
positions are possible. If not, the method continues to step 212, where
the vector having the largest SCORE from the initial analysis is selected
for use in analysis, and template formation is initiated, as indicated at
214. This opt-out step 210 may be provided to accommodate a patient who
is not capable of changing body positions due to physical limitations or
to accommodate use during implant procedures.

[0053] If, at step 210, one or more additional patient positions are
possible, the method continues to step 220. At step 220, the programmer
requests that the patient adopt a different position, for example,
standing or sitting. The method then performs similar steps to those
performed with the first body position. A vector is selected at 222, a
score, VS SCORESTD, is calculated at 224, and it is determined
whether each vector has been considered at 226. If not, as indicated at
228, the method returns to 224 with a different vector selected. Once all
vectors are completed, the vectors are compared, as indicated at 230. In
an illustrative embodiment, the formula shown at 236 is used to calculate
a PASSValue for each vector.

[0054] Formula 236 includes two major terms. FIGS. 6A-6B are graphs of
variable relationships in an illustrative method of analyzing sensing
vectors during postural assessment, with FIG. 6A illustrating the first
term:

When the signal amplitudes are closest to one another, the value of this
term approaches 1, while it approaches zero if the maximum signal
amplitude of the sensing vector when the patient is in one body position
is significantly different from the maximum signal amplitude of the
sensing vector when the patient is in the other body position.

[0056] In short, illustrative formula 236 takes into account the size and
similarities of the SCOREs, i.e., whether the vector quality is high and
the strength of the signal along a given vector is high in the first
term, and whether the vector provides relatively consistent output,
particularly in terms of signal amplitude, without regard for the body
position of the patient. One reason for the inclusion of the second term
is that, in the illustrative embodiment, the event detection system of
the implantable device includes an amplifying input having two dynamic
ranges, one which is larger and one which is smaller. With such a system,
it may be better, in some embodiments, to select a vector that captures
signal that makes relatively full use of one of the dynamic ranges in
each of the patient body positions.

[0057] For example, given dynamic ranges of 0-2.0 mV and 0-4.0 mV,
analysis may be easier and more reliable with a first vector in which
peak voltages are 0.75 mV for both body positions, rather than a second
vector in which peak voltages are 0.75 mV for one body position and 2.5
mV for a second body position, as the latter vector would call for lower
amplification to accommodate the second body position but would use the
dynamic range poorly when the patient is in the first body position. For
example, FIG. 7C illustrates a SCORE calculation in which the amplitude
affects the output SCORE in a manner having first and second peaks, with
a trough therebetween. While having both terms account for dynamic range
is part of some illustrative embodiments, other illustrative embodiments
may instead use the formula of FIG. 7A for score calculation, as further
explained below.

[0058] The illustrative formula 236 presumes a SCORE value is generated in
some manner. Some illustrative embodiments use the following approach:

SCORE=SA×SR

Where, for example, SA and SR may be calculated using one of
several approaches. In one such embodiment, the following formula is
used:

It can be seen that the output scores using the lookup table would
include a trough in the range of 1.7 mV<QRSAvg<2.0 mV, in
accordance with an embodiment adapted for multiple dynamic ranges.

[0064] In yet another embodiment, SA and SR are calculated using
another polynomial formula, for example:

SR=CR*(SNR)2

where:

[0065] if SNR≦3.5, CR=0.1;

[0066] if
3.5<SNR≦10, CR=1; and

[0067] if SNR>10, SR=100 It
can be seen that SR is calculated the same here as in the first
calculation method shown above, and a graph showing the relationship is
shown in FIG. 7B. In this illustrative example, SA may be calculated
using the following:

A graph illustrating the relationship between SA and QRSAVG is
shown in FIG. 7C. Both the lookup table and this third method using a
6th order polynomial take into account a system having first and
second dynamic ranges by providing a dip in the SCOREs corresponding to
input signals that would border between the two dynamic ranges.

[0075] These methods of calculating SCOREs are merely illustrative, and
those of skill in the art will understand that the values and
calculations involved will vary depending upon the positioning of the
system, the electronics and electrodes used, power level(s), and,
potentially, other variables.

[0076] It is sufficient for the present invention that an analysis of a
sensing vector is performed with the patient in two or more body
positions, if two positions are possible, and that this analysis provides
a result that indicates whether or not the vector is useful/usable. For
some embodiments, the analysis may further provide results for several
vectors such that the vectors may be compared to one another. For
example, in the method of FIG. 3, a Boolean output of Yes/No may be a
result of vector analysis such that, if a vector is initially selected
and is functional, a Yes output results. For the method embodiments of
FIGS. 4-5, however, an output of vector analysis that allows comparative
analysis is illustrated, as it allows the vectors to be compared to one
another at the end of analysis. In alternative embodiments, a method as
in FIGS. 4-5 returns Boolean results, with the available vectors
prioritized such that the highest priority vector returning a functional
result (Yes output, for example) is selected.

[0077] Referring again to FIG. 5, after comparison of the vectors at 230,
the vector with the largest PASSValue is selected as the default or
primary sensing vector, as indicated at 232. Template formation may then
be initiated, as indicated at 214, and the device may move on to ordinary
function. If desired, a secondary sensing vector may also be identified
in step 232. A secondary sensing vector may be used, for example, for any
of the reasons set forth above, including to resolve ambiguities, to
provide a Boolean check of cardiac function, or to provide a second
vector to use when the patient changes body position.

[0078] In a more general embodiment making use of similar relationships,
the first formula could be of the form:

where n is the number of body positions tested, and SCOREi is the
score for the vector while the patient is in the ith body position.
The amplitude factor may be analogous to the above amplitude factor. An
additional factor which may be included takes the form:

Min ( SCORE 1 SCORE n ) Max ( SCORE 1
SCORE n ) ##EQU00004##

This term would further emphasize a minimum score value, for example,
when using three body positions in the postural assessment, if a very low
score is achieved in any of the positions, the vector under consideration
may be poorly suited to detection regardless of high SCOREs in the other
positions, and this factor would reduce the output PASS value.

[0079] FIGS. 8A-8E are graphical representations of display outputs of a
programmer during an illustrative method of postural assessment. In the
illustrative embodiment, the programmer includes a touch screen; in other
embodiments, the programmer may include buttons, a keypad or other
controls, and may take any suitable form.

[0080] FIG. 8A shows a first screen shot. The programmer indicates to the
physician that the postural assessment procedure is to start. The
physician is asked to ensure that the patient is laying down (a first
body position) and then to touch the "continue" icon 300 on the screen.
Across the top of the programmer screen, status is indicated for the
implanted device. In particular, during these steps, therapy for the
device may (optionally) be turned off. The device status as "implanted"
is indicated, as is the patient's heart rate. In some embodiments, the
programmer may refuse to perform postural assessment if the patient's
heart rate is not in a predetermined range, for example, between 50 and
120 bpm.

[0081] After the physician presses the "continue" icon 300 from the screen
shot of FIG. 8A, the next screen that is seen is that of FIG. 8B, in
which the programmer indicates that the device is collecting the
patient's rhythm. The physician is requested to keep the patient still. A
cancel icon 302 is provided in case the physician determines that the
procedure should stop, for example, if the patient feels uncomfortable or
ill, displays a physical abnormality (rising heart rate, for example), or
if the patient moves. If desired, a status bar may be provided to
indicate the progress of the sensing vector analysis to the physician.

[0082] Once data is captured for the first patient body position, the next
screen shot is that of FIG. 8C. The physician is asked to have the
patient sit up (a second body position). With the patient sitting up, the
physician is asked to keep the patient still and depress the continue
icon 304. The programmer then displays the screen shot of FIG. 8D, which
is quite similar to that of FIG. 8B and again includes an optional cancel
button 306 and may include a status bar.

[0083] As shown in FIG. 8E, once data capture is complete for the second
body position, patient tailoring in the illustrative embodiment is
complete. The physician (or other operator) may go on to perform other
tasks by touching the "continue" icon 310. In other embodiments,
additional data capture may ensue, if desired.

[0084] In some embodiments, the data capture for the patient may include
options for physician input. For example, during data capture, it is
possible for an artifact (such as a T-wave) to interfere with detection
of R-waves. In some instances, the physician's input may be needed or
requested to resolve any questions relating to event classification. Some
examples are discussed in copending U.S. patent application Ser. No.
11/441,522, filed May 26, 2006 published as US Patent Application
Publication Number 2007-0276445 and entitled SYSTEMS AND METHODS FOR
SENSING VECTOR SELECTION IN AN IMPLANTABLE MEDICAL DEVICE. In one
illustrative embodiment, any such questions are delayed until all data
has been captured, allowing the physician to concentrate on the
programmer to answer such questions, rather than having the patient
remain in a predetermined position while such issues are resolved. In
another illustrative embodiment, such questions are asked as they arise.

[0085] In some embodiments, the above methods may be revisited later,
after implantation, by the patient on his or her own. For example, as
home-monitoring systems become available for patients with implanted
cardiac monitoring and/or stimulus devices, a home monitoring system may
be used to communicate with the implanted device, allowing later
re-selection of sensing vectors in light of postural assessment. The home
monitoring system, in an illustrative example, may include a graphical
user interface allowing the user to indicate readiness for postural
assessment, after which the home monitoring system may provide graphical
output indicating what to do, physically, for the patient to complete
home-monitoring self-assessment of postural effects on cardiac signals.
Thus, a home monitoring system having functionality allowing it to
provide patient directions in support of postural assessment may also be
a "programmer" in the methods and systems discussed herein.

[0086] Yet another illustrative embodiment may include a device as shown
in either of FIGS. 1A-1B which includes a sensor or sensing system for
determining patient body position. For example, the sensor may be a
gravity sensor or accelerometer-type sensor. The sensor may also be used
to measure transthoracic impedance as a surrogate for patient body
position. In one embodiment, the system need not determine an actual
position using the position sensor, but instead may identify position
sensor output ranges that correlate to the usefulness of particular
vectors and/or templates. For example, when a patient moves from sitting
to standing, a position sensor output may indicate a change of position.
When the position changes as indicated by the position sensor, the system
may determine for itself whether the primary sensing vector should be
changed by observing various template analyses. To this end, from the
implanted medical device system perspective, knowledge of the position is
not needed, but instead, identification of the best template from several
that are available is sufficient.

[0087] During operation, another illustrative implanted medical device may
periodically (at intervals) or occasionally (in response to a condition
or request) perform postural assessment without requesting movement by
the patient. For example, if the position sensor output is "X", vector
selection may be performed. If the position sensor later provides a
different output, "Y", vector selection may again be performed, as is may
be presumed that the patient is in a different body position. This
process may be repeated several times, with templates and vectors
identified for various position sensor outputs. After the selection
process, if the position sensor output returns to "X", then a vector
and/or template associated with position sensor output "X" may be
selected. Within this approach, a single vector may have multiple
templates, each corresponding to a position sensor output.

[0088] Those skilled in the art will recognize that the present invention
may be manifested in a variety of forms other than the specific
embodiments described and contemplated herein. Accordingly, departures in
form and detail may be made without departing from the scope and spirit
of the present invention as described in the appended claims.